1. Field of the Invention
The present invention relates to a fibrillation detector that processes an electrocardio signal obtained from a heart of a living body, and a defibrillator including the fibrillation detector.
Priority is claimed on Japanese Patent Application No. 2012-238724, filed Oct. 30, 2012, and published as Japanese Publication No. 2014-087454 (A) on May 15, 2015, the content of which is incorporated herein by reference.
2. Description of Related Art
When a heart normally operates, the heart enters a state called a normal sinus rhythm (NSR). Hereinafter, characteristics of the electrocardio gram (ECG) in the NSR state will be described with reference to FIGS. 9 and 10. FIG. 9 shows a diagram cited from FIG. 4.4 of John G. Webster, “Design of Cardiac Pacemakers”, IEEE Press (1995), and FIG. 10 shows a diagram cited from FIG. 8.6 of John G Webster, “Design of Cardiac Pacemakers”, IEEE Press (1995).
When an ECG in the NSR state is measured, an ECG called QRS waves as shown in FIG. 9 is obtained. FIG. 9 shows a waveform of the ECG In this drawing, a horizontal direction represents time, and a vertical direction represents amplitude. Characteristics of P, Q, R, S, T, and U described in FIG. 9 represent signals called a P wave, a Q wave, an R wave, an S wave, a T wave, and a U wave in the ECG, respectively.
FIG. 10 shows results obtained by performing a frequency analysis (Fourier transform) of the ECG in the NSR state. From a graph in FIG. 10, it can be seen that the center of a spectrum of the T wave appears at approximately 5 Hz, and the center of a spectrum of the R wave appears at approximately 8 to 15 Hz.
For a patient to which an implantable cardioverter defibrillator (ICD) is applied, a cardiac seizure called a ventricular fibrillation (VF) may occur. When the VF occurs in a patient, the ECG of the patient shows an irregular amplitude or waveform, and as described later, it is difficult to distinguish the QRS waves or T wave. In a case of performing a Fourier transform of the ECG in the VF state, an amplitude of a spectrum, which is present at approximately 8 to 15 Hz and corresponds to the R wave, has a tendency to decrease, and an amplitude of a spectrum, which is near approximately 5 Hz and corresponds to the T wave, has a tendency to increase. The VF causes a circulatory arrest of blood within several seconds, and is the largest cause of sudden cardiac death. Therefore, it is necessary to immediately perform an electrical defibrillation.
Hereinafter, description will be made with respect to a structure in which the ICD described in the U.S. Pat. No. 5,891,169 detects a cardiac beat with reference to FIGS. 11A, 11B and 11C and 12A and 12B. FIGS. 11A, 11B and 11C show a diagram corresponding to FIG. 2 of the U.S. Pat. No. 5,891,169, and FIGS. 12A and 12B show a diagram corresponding to FIGS. 3A, 3B and 3C of the U.S. Pat. No. 5,891,169.
FIGS. 11A, 11B and 11C illustrate a method of measuring a cardiac rate using a fixed threshold value. In this drawing, a horizontal direction represents time, and a vertical direction represents amplitude. In this drawing, a solid line represents an ECG, and a broken line represents a threshold value. A portion at which the solid line and the broken line intersect each other is counted as a cardiac beat. The solid line in FIG. 11A represents an ECG of a typical NSR, the solid line in FIG. 11B represents an ECG of a typical ventricular tachycardia (VT), and the solid line in FIG. 11C represents an ECG of a typical VF. In addition, broken lines TI, TII, and TIII represent threshold values that are very suitable to detect cardiac beats of the NSR, VT, and VF, respectively.
In general, when the counted cardiac rate is 145 bpm (beats per minute) or less, it is diagnosed as NSR, when the counted cardiac rate is 146 to 225 bpm, it is diagnosed as VT, and when the counted cardiac rate is 226 bpm or more, it is diagnosed as VF. In addition, when peak values of respective ECGs of the NSR, VT, and VF are compared to each other, the peak value of the VF apparently has a value smaller than those of the NSR and VT. As can be seen from FIGS. 11A, 11B and 11C, when a high threshold value, which is very suitable to detect the cardiac beat of the NSR and VT, is set, the cardiac beat of the VF is not detected, and when a low threshold value, which is very suitable to detect the cardiac beat of the VF, is set, there is a danger that the T wave that is present in the ECG of the NSR and VT is detected as the cardiac beat.
Therefore, as indicated by a broken line in FIG. 12A, a method of detecting the cardiac beat using a threshold value that exponentially attenuates from a peak position of the R wave (the solid line at an upper end of FIGS. 12A and 12B) is generally used. This method is called an AGC method. FIGS. 12A and 12B illustrate a method of measuring the cardiac rate using the AGC method. A solid line in FIG. 12A represents an ECG and a broken line in FIG. 12A represents a threshold voltage in the AGC method. In the drawing, a horizontal direction represents time, and a vertical direction represents amplitude. In a general AGC method, the threshold value has a characteristic of exponentially decreasing from a 75% value of a peak value when detecting the R wave, and a time constant thereof is 400 ms.
A bar graph of FIG. 12B represents a timing at which a cardiac beat is detected, and a portion of a range A in FIG. 12B corresponds to the NSR, a portion of a range B corresponds to the VT, and a portion of a range C corresponds to the VF. As can be seen from this drawing, the cardiac rate in NSR and VT sections is correctly detected.